The field of the invention is magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to systems and method for controlling repetition time dependencies with respect to T2-weighted MRI imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins,” after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the transverse magnetic moment Mt. The amplitude, A, of the emitted NMR signal decays in an exponential fashion with time, t.
An important factor that contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process that is characterized by the time constant T1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z-magnetization). The difference in T1 between tissues can be exploited to provide image contrast.
The decay constant 1/T*2 depends on the homogeneity of the magnetic field and on T2, which is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B1 in a perfectly homogeneous field. The T1 time constant is longer than T2 and, in fact, the T1 time constant is much longer than T2 in most substances of medical interest.
The practical value of the T2 constant is that tissues have different T2 values and this can be exploited as a means of enhancing the contrast between such tissues. Accordingly, T2 serves as a basic, but very informative MRI parameter, providing non-invasive measurements of tissue status and disease prognosis with respect to a wide range of applications and a host of diseases, including epilepsy, multiple sclerosis (MS), stroke and tumor. In addition, quantitative T2 mapping offers tremendous insights into brain development, iron deposition, and metabolism.
In order to quantify T2, multiple T2-weighted images are acquired and fitted against their echo time (TE), assuming long repetition time (TR) for complete relaxation. In practice, however, a short TR is often desired to minimize scan time. Thus, when looking to quantify T2 and T2-related parameters, the desire to minimize scan time may undermine quantitative T2 measurement. For instance, in a recent study of child-brain development, T2 measurements were found to be two to four times larger than those found in an earlier study and the discrepancy between these findings were at least partially attributable to the choice of different TRs.
Accordingly, given the particular value and versatility of T2 measurements in MRI and the substantial need to minimize scan time, which is in stark competition with traditional mechanisms for optimizing T2-based contrasts and quantifications, it would be desirable to have a system and method for controlling the scan-time dependence of T2-weighted MRI imaging.